Serial printers are printers of the type having a number of printing elements less than is necessary for printing across an entire line. In the operation of a serial printer, the print element moves over a certain distance along a line and is actuated to effect printing at a plurality of locations during its travel. There are two types of serial printers; fully formed character printers and matrix printers. Fully formed character printers print the entire character in one print operation; an example would be the popular daisy wheel printers. Serial matrix printers, in general, print by composing a character by means of the actuation of a plurality of wires, ink jets, heating elements or other actuating means that print less than an entire character so that the characters are composed of elements such as dots. This invention is useful with either fully formed character or matrix printers.
Originally serial printers printed from a left position or starting point and sequentially printed until the line was complete. At the completion of the print line, the paper was indexed and the print element was returned to the starting point before printing the next print line. To increase through-put, it is known to print bidirectionally; that is, the next print line is printed backwards instead of returning the print element to the starting position. U.S. Pat. No. 3,708,050 to McCarthy for Printer Control With Monodirectional and Bidirectional Printing Compatibility describes a printer that can accept a data stream formatted for a monodirectional printer yet print the data bidirectionally. U.S. Pat. No. 4,463,444 to Daniels et al. for Word Processing System Having A Formatting Bidirectional Printer is a further enhancement.
In order to further increase through-put of bidirectional printers, it is known to position the print element based on stored information relative to the right and left margins of the next print line. At the completion of a print line, a comparison of the current print line end position is made with the right and left end positions of the next print line to determine the closest end position. On the basis of that distance comparison, the print element is directed to proceed to the closest end position of the next print line. This technique also known as bidirectional logic seeking, is fully explained in U.S. Pat. No. 3,764,994 to Brooks et al. for Serial Printer With Bi-Directional Drive Control.
A further improvement to the bidirectional logic seeking printers is described in U.S. Pat. No. 4,376,588 to Moeller for Bi-Directional Serial Printer With Look-Ahead. The conventional bidirectional logic seeking printer described by Brooks et al., is modified to determine the distance between the end position of the current print line and the beginning print position of the next print line in the direction of print element travel. If the determination indicates that the distance is less than a specified maximum distance, the print element is continued in the same direction past the end print position of the current line to the beginning position of the next line. This determination must be made prior to the print element reaching the end position of the current print line. Looking ahead to the next print position, prior to stopping the print element, and continuing the print element in the current direction if that distance is small, increases over all printer through-put.
In U.S. Pat. No. 4,469,460 to Hughes et al. for Matrix Printer With Optimum Printing Velocity a technique is described for further enhancing printer through-put by selecting the print element velocity as a function of print line length, so that for short line lengths, a low velocity is selected. By selecting a low velocity, less time is spent accelerating and decelerating the print element. For short enough line lengths the time lost printing at a lower velocity is compensated by the time gained in not having to accelerate to a high velocity and decelerate therefrom; thus, printer through-put is enhanced. For longer print lines, it is worth accelerating the print element to the maximum constant speed.
A disadvantage of the foregoing techniques has been the inability of printers to print at the theoretical limit set by the repetition rate of the print element. In an impact printer for example, the repetition rate is determined by the minimum time between print actuator firings. Some of the factors limiting the printer through-put are (1) the time required to index the record medium, (2) the turn-around settling time, (3) the time to accelerate to constant velocity and (4) the time to decelerate from constant velocity.
One approach to overcoming these disadvantages is described in an article entitled Print Head Speed And Stopping Distance Specification For Voltage-Drive Motor Control Systems disclosed in IBM Technical Disclosure Bulletin, Vol. 29, No. 6, November 1986, page 2652. The technique described permits printing on the acceleration and deceleration ramps, however, accurate position sensors and complicated feed back systems are required.